1. Technical Field of the Invention
The present invention relates to a stator cascade of a turbo type fluid machine such as a fan, a compressor, a turbine or the like.
2. Description of the Related Art
FIG. 1 is a view schematically showing a layout state of conventional rotor cascade 2 and stator cascade 4, for example, in a turbo type fluid machine such as a fan, a compressor, a turbine or the like of a jet engine.
In this drawing, a vertical direction corresponds to an axial direction and a horizontal direction corresponds to a peripheral direction. As shown in this drawing, the rotor cascade 2 is constituted by a plurality of rotor blades RB1 to RB5 which are arranged at an equal interval in a peripheral direction, whirls in a direction shown by an arrow in the drawing, and feeds a fluid such as an air or the like to a downstream side. The stator cascade 4 is arranged in a downstream side of the rotor cascade 2, is constituted by a plurality of stator blades SV1 to SV7 which are arranged at an equal interval in a peripheral direction, and commutates the fluid from the rotor cascade 2 so as to circulate further downstream.
In this case, if the rotor cascade 2 mentioned above is swirled, a following wake (also called as a wake, hereinafter, refer to as “wake”) having different speed components in an axial direction is generated in a rear end portion of each of the rotor blades RB1 to RB5 in a trailing manner. Further, since the wake is moved in the peripheral direction together with the rotor blade, a periodical interference is repeated between the wake and each of the stator blades SV1 to SV7, and a great pressure fluctuation is generated on a surface of the stator blade due to the interference. The pressure fluctuation is a fluctuation having a frequency (hereinafter, refer to as “rotor blade passing frequency”) obtained by multiplying a number of the rotor blades constituting the rotor cascade 2 by a rotational speed (rpm), and a noise having the rotor blade passing frequency mentioned above is generated by the pressure fluctuation. The noise generated as mentioned above is called as “interference noise”.
In a jet engine, since a jet noise is widely reduced on the basis of an appearance of a high bypass ratio turbofan engine, a main body of the engine noise is changed from the jet noise to a fan noise, and a main sound source of the fan noise is constituted by the interference noise mentioned above. Accordingly, in the light of an environmental suitability, it comes to an important problem to reduce the interference noise generated in the fan, particularly in the turbofan engine.
As one of prior arts for reducing the interference noise, there has been known a method of selecting a ratio of number between the rotor blades and the stator blades optimum. This is a method of selecting the rotor stator blade number ratio in such a manner that a specific component in frequency components (constituted by a primary component, a secondary component, a tertiary component, . . . an nth component) of the propagated noise attenuates exponentially. The attenuation of the specific component is called as “cutoff”, and a condition at that time is called as “cutoff condition”.
In the interference noise, since a sound level of the primary component is highest in the frequency component of the interference noise generated by the rotor blade passing frequency mentioned above, the rotor stator blade number ratio is generally selected in such a manner as to cut off the primary component. The interference noise is reduced by attenuating the primary component of the interference noise in the manner mentioned above.
In this case, with regard to the reduction of the interference noise, various proposals have been made in the following patent documents 1, 2 and the like.
Further, a generating mechanism of the interference noise in the fan of the jet engine is described in the following non-patent document 1.
Patent Document 1: Japanese Unexamined Patent Publication No. 2002-349498
Patent Document 2: Japanese Unexamined Patent Publication No. 2003-227302
Non-Patent Document 1: T. G. Sofrin, “Aircraft Turbomachinery Noise Fan Noise”, Pratt & Whitney Aircraft Jul. 31-Aug. 10, 1973
In accordance with the optimum selection of the rotor stator blade number ratio mentioned above, it is possible to obtain a certain level of noise reduction effect. However, this method only cuts off the primary component of the noise, and can not cut off the higher order component equal to or more than the secondary component. Accordingly, in order to suppress a strength of the interference noise generated by the higher order component equal to or more than the secondary component, it is necessary to employ a countermeasure of attaching a sound absorption plate or enlarging an interval in an axial direction between the rotor blade and the stator blade.
However, the following problems are generated in these countermeasures.
In accordance with the attachment of the sound absorption plate, the temporarily generated noise is absorbed by the sound absorption plate. However, for example, in the case of the jet engine, since the sound absorbing plate is required at about 70 kg for reducing the noise at 3 dB from an actual condition, there is generated a problem that an increase of a cost and an engine weight is caused.
In accordance with the enlargement of the rotor stator blade interval, it is possible to promote the attenuation of the wake accompanying in the rotor blade rear end portion, and it is possible to weaken a non-steady aerodynamic force on the stator blade surface forming the noise source. However, for example, in the case of the fan of the jet engine, if the interval in an engine axial direction between the fan rotor blade and the fan stator blade is enlarged, the length in the engine axial direction is elongated, so that there is generated a problem that an increase of the engine weight is caused.